1. Field of the Invention
The invention relates to paper machine head boxes having a plurality of channels at a downstream portion thereof defined by spaced lamellae, and in particular to the geometry of the lamellae.
2. Description of Related Technology
It is known to dispose lamellae (i.e., thin sheets or plates) in a multi-layer headbox at a downstream portion thereof with respect to the direction of flow of pulp through the headbox. The lamellae separate the pulp streams of a multi-layer headbox in such a way that mixing of the streams from the different layers does not occur until the streams are combined to a single stream near an exit opening of the headbox. The single stream of pulp then flows from the exit opening to a forming surface of a paper making machine.
The lamellae reduce turbulence in the flow channels defined thereby and provide pulp to the paper making machine forming surface in a form as undisturbed as possible. However, flow turbulence frequently originates at the downstream ends of the lamellae. Various attempts have been made to avoid such disturbances in the pulp flow. For example, German Patent Application DE 43 23 050 A1 discloses a lamella design that produces an increased flow convergence in the flow channel, resulting in an increase in flow velocity in this area, providing a reduction of disturbances due to turbulence.
Generally, turbulence at the end of a lamella can be reduced by tapering the end to a sharp edge. However, as a practical matter, a lamella cannot be made very sharp for reasons of manufacturing technology, cost and operational safety. As a consequence, because of the lamella end thickness, turbulence or periodic separated occurrences of turbulence occur which subsequently transfer vibration to the lamella end and thus to the entire lamella. As a result of this vibration, disturbances occur in the formation of the individual layers of pulp.
German Patent Application DE 29 16 351 (corresponding to Stenberg, U.S. Pat. Nos. 4,349,414 and 4,445,974) proposes to solve the lamella turbulence problems by providing lamellae with capillary ends which introduce air into the pulp stream and thus reduce turbulence. Such a headbox is apparently expensive to manufacture since the lamellae must be bored with a large number of small capillaries.